The present invention is directed to a method and system for controlling a motor. In particular, the present invention is directed to controlling the vibration of a motor by controlling the flux producing current in the motor.
All electric motors experience some type of mechanical vibration. Mechanical vibration has been known to be a function of the speed of the motor as well as the resonant frequency of the motor structure.
Previous attempts to control motor vibration have included structurally revising the motor. This approach includes revising the stator core and frame to alter the resonant frequency and response of the motor. Another approach involves changing the stator core winding to operate with a different flux density and to reduce any flux asymmetries. A problem with this approach is the increased manufacturing and design costs of these alterations. Also, customizing and/or maintaining the mechanical tolerances of the motor is not often possible on a customized basis without incurring undue expense.
Another approach to this problem involves injecting a single phase current at a motor resonant frequency to act as a counter balance for flux asymmetries in the motor that are causing the vibration. Thee phase and amplitude of such an injected single phase current are determined by running the motor at the highest point of vibration and adjusting the phase and amplitude values until the lowest measured vibration level occurs. The second approach is limited by the response capabilities of the drive torque regulator that is applying the injected current. Additionally, such an injected current may create an undesirable h torque ripple that may affect the stability of outer speed and torque regulation control loops.
The present invention controls vibration by altering the flux profile of a motor. The inventor understands that torque is created as a vector product of the applied flux producing and torque producing currents. The inventor came to the realization that, in some cases, motor vibration is largely dependent upon a flux producing current. The invention involves profiling the flux producing current throughout the operating speed range of the motor to control the motor vibration.
An embodiment of the invention controls the motor vibration while developing the rated torque of the motor. For example, if a motor vibration exceeds a desired level, the flux producing current may be reduced to reduce the motor vibration until the vibration falls below the desired level of vibration. For a variable speed motor, motor vibration may also be a function of the speed of the motor. In this instance, the motor vibration may only exceed a desired level at a given speed. In this case, the present invention reduces the flux producing current only within the speed range where the excessive vibration occurs. Again, the flux producing current is reduced in this speed range (or ranges) until the vibration level falls below the desired level. The torque producing current may also be increased to maintain the motor torque that may result from a reduction of flux producing current.